Designing the biggest fasting app on earth
How It Started
I'd worked with Zero's founder before at FitStar, which Fitbit acquired. When he told me about Zero, I'd just gotten into fasting myself and had just moved from the Bay Area to Brooklyn — a remote role at an early-stage consumer app was exactly what I wanted. When I joined, I was one of the first three people besides the founder — a full-stack engineer, a product guy, and me. The product was a numerical countdown timer. No accounts, no profiles, no subscription. People would download it, start a fast, and watch a number tick down. Organic growth was strong because intermittent fasting was exploding, but retention was terrible. Users would track for a few weeks and disappear. There was nothing to come back for once the novelty wore off.
Making the Timer Matter
The timer was the product, so the timer had to evolve first. We made it visual — a round display that actually felt like something was happening during your fast. That mattered because fasting is boring. You're not doing anything. The app's job was to make the not-doing-anything feel purposeful. The biggest unlock was fasting zones. Fasting phases don't work like clean start and end points. The phases overlap, they depend on the person, their genetics, the length of the fast, and they shift throughout. I wanted to capture that in the timer itself — visual zones that represented what's actually happening in your body at each stage, from glucose depletion through fat burning to autophagy. As the timer progressed, you'd move through zones and understand why you were still going. It turned the timer from a countdown into a story about your body, and it became the single biggest driver of Pro conversions by a wide margin.
Building the Platform
We built out the rest of the platform around it — user accounts, onboarding, subscription flows, content, history, social features. The accounts retrofit was rough. Try getting thousands of existing users to create accounts to access their own data. We tried to make it clear this was the only way to save their history across devices, but we lost some users over it. Zero became the world's most popular fasting app. Millions of users, 4.8 stars from 445K ratings. Elon Musk publicly credited intermittent fasting for his weight loss in 2022 and the app got a wave of press behind it.
Looking Back
If I started Zero over, I'd think about free vs. paid from day one. We'd made the free experience a little too good, so converting users to Pro was a constant uphill battle. You can't lock previously free features behind a paywall without a revolt, which means you have to invent new features worth paying for — and that's hard when the core free loops already solve most people's main use case. That tension defined a lot of the product strategy, and it would've been easier to navigate if we'd drawn the line earlier.